voltage switching help

How can I do this: I have (2) input dc voltages (input a and input b). When (a) is higher then (b) My output will be +vdc. If (b) is higher then (a) My output is zero or open. Im not very good at this so I'll try using numbers. Im looking for a device or component that will send voltage to a 12 vdc relay. The relay turning on and off is based on (2) input voltages. As long as 1 voltage is higher than the other my relay will close. So lets say if input volatage (a) = 5vdc and input voltage (b) = 10 vdc, a 12 volt source will turn a relay on. If (b) is less then (a) no voltage will turn on relay. I have very little experience in electronics any detail/ help would be great...Thank You. By the way can an op amp do what im asking?